Studies in dogs will be directed toward understanding mechanisms of the hepatic excretion of biliary contrast materials (BCM). The currently available BCM will be studied (iopanoic acid, tyropanoic acid, iocetamic acid, sodium ipodate and methylglucamine iodipamide). The bile flow, biliary concentration, total biliary excretion and the plasma concentration of the BCM will be examined under a variety of circumstances to determine: 1) the relation of plasma concentration to hepatic excretory efficency of BCM, 2) the role of bile acids and bile flow in the hepatic excretion of BCM, 3) the presence of competition between different BCM and between BCM and other compounds that are excreted in bile, 4) the influence of phenobarbital, estrogen and hypertonic glucose on hepatic excretion of BCM, 5) the volume and type of choleresis associated with hepatic excretion of BCM, 6) the effect of BCM on the excretion of cholesterol, lecithin and bile acids in bile and 7) the effect of compounds that alter ductular bile flow on BCM excretion. In addition, factors effecting the intestinal absorption of BCM and the binding of BCM to hepatic plasma cell membranes will be studied. This information is relevant to an understanding of the cause and prevention of lithogenic bile and to the cause of the cholestatic syndromes in humans. It is necessary for the rational development of practical methods to improve clinical cholecystography and cholangiography.